The invention is directed to gyrotron traveling wave tube amplifiers (gyro-TWTs), and particularly to arrangements for preventing oscillations near the cutoff frequency of the waveguide interaction structure in such gyro-TWTs.
The gyro-TWT is a relatively new type of amplifier in which an electron beam with a large component of velocity transverse to the axial direction of travel interacts with the transverse components of RF electric field in a waveguide to provide amplified power output. To obtain maximum gain and power, the value of magnetic field should be adjusted so that the relativistic cyclotron frequency is slightly below the waveguide cutoff frequency or to a subharmonic of this frequency. This is prevented by a tendency for parasitic oscillations to occur in the range of the waveguide cutoff frequency, where both forward and backward wave interaction can occur and where the low group velocity causes the interaction impedance to be very high. Present methods to reduce oscillations include (1) detuning the magnetic field to avoid synchronism with the waveguide cutoff frequency, (2) reducing the beam current, (3) reducing the transverse velocity, and (4) use of lossy coatings such as carbon or Kanthal (an Fe-Cr-Al-Co alloy) to increase circuit RF loss. All of these present methods have features which limit the gain, power, and efficiency.